This invention relates to a method of rolling a plate, billet, bar, or rod made of a metal workpiece, and particularly relates to a method of hot rolling of a metal workpiece which is capable of being rolled into a plate, billet, bar or rod stably with a large reduction of area or rate of elongation per pass.
In a conventional rolling operation wherein the reduction of area per pass is within 20-30%, in order to determine a maximum level of the reduction of area, a contact angle .theta. between the workpiece and a workroll must be considered. Namely, in any condition of rolling speed, roll workpiece and the like, in order to perform stable rolling, the contact angle .theta. must be in the relation of .theta.&lt; tan.sup.-1 .mu.(.mu. denotes the coefficient of friction between the workpiece and workrolls during biting), and it is common practice to determine a maximum level of proper reduction of area within the range of such contact angle .theta..
On the other hand, with respect to the coefficient of friction between the workpiece and workrolls, assuming that the coefficient of friction in the steady rolling condition after the biting is completed is .mu.', the following relation is well known: EQU tan.sup.-1 .mu.'&gt; tan.sup.-1 .mu.
From this relation, in the stationary rolling condition, rolling with a large contact angle as compared with the contact angle at the time of biting, namely, rolling with a high reduction of area in the range of .theta..gtoreq. tan.sup.-1 .mu. is possible.
A method of rolling a metal workpiece with a high reduction of area is disclosed in U.S. Pat. No. 4,106,318. In this method a metal workpiece is rolled by a rolling mill wherein the workrolls are supported so that the rolls do not shift in the advancing direction of the workpiece, and a gap between the workrolls is so adjusted to keep the contact angle .theta. in the relation of .theta..gtoreq. tan .mu., and the rolling is performed while the workpiece is continuously pushed between the rolls having the foregoing gap, with a of pushing force of a magnitude such that a neutral point of the rolling is caused to exist in the plane where the workrolls and the workpiece contact. However, in this method, there is required a pushing device such as a large scale pusher for pushing the workpiece continuously with stability over the entire length of the workpiece between the workrolls. Therefore, there is the drawback that the continuous rolling with the high reduction of area per one pass over a plurality of stands is extremely difficult. As the pushing device for eliminately this drawback, for example, a masterslave pusher system is available. In such device, to the first stand, the workpiece is pushed in by a master pusher, and to the second stand, the workpiece is pushed in that pass through the first stand. However, in this device, in order to push the workpiece through the workrolls continuously, two passes is a limit, and pushing through more than two passes is substantially impossible. Accordingly, there is a limit in the effect of making the rolling mill train into a compact size by employing such high reduction rolling system. Furthermore, as a method of pushing the workpiece, there is a method of pushing the workpiece into the roll gap by means of pinch rolls or of the rolling mill itself, and in these methods, the foregoing problems arising from the use of the pusher apparatus can be eliminated. However, when the pushing force becomes zero between the pinch rolls and the rolling mill or between the rolling mills, the workpiece is rolled under a non-stationary condition. Because of this phenomenon, there is the danger of defective biting, and also greater fluctuation in the dimensions of the width direction occurs, resulting in adverse influences with respect to the rolling operation, yield and product quality. In order to solve such problems, the rolling operation may be performed by connecting the preceding and succeeding workpieces by means of welding, but for this operation, another installation becomes necessary, thus extremely complicating matters related to technical and installation aspects. Also, the pushing force applied to the workpiece exerts an influence over the deformation of the workpiece between the workrolls, and as a result, spreading in the width direction over the entire length of the workpiece is increased, which gives rise to the deterioration of deformation efficiency.
As an example of a method of rolling a workpiece with a high reduction of area where the contact angle .theta. becomes .theta.&gt; tan.sup.-1 .mu., the method of U.S. Pat. No. 3,553,997 is known. This rolling method is a method of performing an in-line reduction in a continuous casting wherein a front end of the workpiece is made to pass between the workrolls to a certain extent by opening the gap of the workrolls sufficiently and the workpiece is gripped by the workrolls. Then, the roll gap is made smaller to perform the rolling with the large contact angle. In this rolling method, it is possible to roll the workpiece with the high reduction of area, but the tip portion of the workpiece is off gauge, thus lowering the yield. This method has other problems such as that the roll gap has to be changed for every piece of the workpiece, and the rolling installations become complicated and large in size. Furthermore, in this rolling method, the cross section of the tip portion of the workpiece is larger than the cross section of the latter part thereof, and also it changes rapidly whereby a dynamic movable guide is required at the incoming side of the rolling stand of the next stage, and the rolling speed has to be changed according to the change of cross section, thus requiring a complicated control mechanism. Also, in this rolling method, in case of groove rolling, the front end portion of the workpiece whose cross section is large does not fit the groove, and thus the rolling of a billet, bar, and rod is almost impossible.
Heretofore, as a method of assisting the biting of the workpiece, a method of cutting the tip of the workpiece in wedge form or a method of pushing the workpiece between the workrolls by causing another cold or hot workpiece to collide with acceleration against the rear of the workpiece have been employed. However, these methods are employed in normal rolling operations wherein the contant angle .theta. is in the relation of .theta.&lt; tan.sup.-1 .mu.. Moreover, in these methods, there are too many problems with respect to yield, maintenance of the installation, stability of operation, and the like, and therefore such method are not practical.